Function validate_optional 

pub fn validate_optional(
    rule: &ValidationRule,
    field_value: &str,
    field_name: &str,
    is_present: bool,
) -> Result<()>

Validates a rule only if the field is present.

If the field is absent/null, validation is skipped (passes). If the field is present, the rule is applied normally.

Arguments

• rule - The validation rule to conditionally apply
• field_value - The value being validated
• field_name - Name of the field for error reporting
• is_present - Whether the field is present/non-null

Errors

Returns FraiseQLError::Validation if the field is present and the rule fails.